In conventional valve lift controllers, several types of actuators are used to linearly drive a shaft of a changing mechanism which controls a lift amount of a valve based on a position of the shaft in its axial direction. For example, an actuator is described in US-2004-0083997A1 (JP-2004-150332A) which converts, by means of a reduction mechanism and a cam mechanism, a rotational driving force of a motor unit into a linear driving force and applies the linear driving force to the shaft of the changing mechanism.
However, the conventional actuator has to use the reduction mechanism in combination with the cam mechanism to make the linear driving force larger. It is therefore difficult to design the actuator to be small. Thus, positions where the actuator can be installed are limited.
The inventors of the present invention have studied a structure of a feed screw mechanism which converts a rotational movement of a rotation spindle to a linear movement of a screwed shaft. The feed screw mechanism can generate a strong linear driving force by means of a simple structure in which the rotation spindle and the screwed shaft are coaxially connected directly or indirectly. An actuator with the feed screw mechanism therefore can be designed to be smaller than the actuator with the reduction mechanism and the cam mechanism.
In the case that a motor rotor is press-fixed on the rotation spindle, the rotation spindle may be deformed, so that a faulty operation of the feed screw mechanism may arise.